With the increasing scale of the commercial application of the Next Generation Network (NGN), reliability of NGN devices shall be guaranteed sufficiently. Since subscriber traffic is undetermined and processing capability of a device is limited, a situation where the traffic brings a load beyond normal processing capability of the device, i.e. an overload, may occur frequently. If the overload results in a partial or complete malfunction of the device, services may be interrupted. Consequently, overload control capability of a device, i.e. ability to operate under overload condition, is a crucial factor with influence upon reliability of the device and even reliability of the network.
The NGN includes a Media Gateway Controller (MGC), a Media Gateway (MG), a Signaling Gateway (SG), a relevant application server, etc. The media gateway includes an Access media Gateway (AG) and a Trunk media Gateway (TG). The AG provides a subscriber side interface connected with subscriber end equipment such as a telephone, terminates signaling of subscriber line and interacts with the MGC through H.248 or MGCP protocol to convert a media stream. The MGC is responsible for call control and controls the AG through the H.248 or MGCP protocol to set up and release a media stream connection.
The call processing capability of the AG and the MGC are limited due to restriction factors such as the CPU operation speed of a system and the like. The system shall be designed in a way that the call processing capability of the system can satisfy a demand for a normal traffic load. For instance, if one AG is provided with a capacity of 2000 line POTS subscribers, each subscriber has an average traffic of 0.1 erlan and each call lasts for 60 s in average, then the AG has an average traffic of 200 erlan and the call processing capability needs to reach 3.3 CAPS (Call Attempts Per Second), that is, 3.3 calls are processed per second. However, in many cases such as in holiday or busy period of a day, the subscriber traffic is typically far beyond 0.1 erlan and thus exceeds the normal processing capability of the system. In order to ensure normal operation of the system and a normal service under a large traffic load, an overload control mechanism is provided for a device to prevent the system from breakdown due to an impact of the large traffic and to guarantee the service as far as possible, in addition to a margin reserved in the system design to satisfy a demand for a high load (for instance, if the average traffic for a subscriber is 0.1 erlan, the system is designed in terms of 0.2 erlan).
The overload control for an AG device is actually to perform restriction for subscriber calls when the load of the system reaches a threshold, so as to make the load of the system resume to a normal level as soon as possible and thereby prevent the breakdown of the system. Such an approach for restricting subscriber calls will necessarily influence the subscriber service during an overload. How to minimize the influence upon the subscriber service depends upon the selection of call restriction policy. A subscriber coupled to an AG device may be an ordinary resident subscriber, or may be a commercial subscriber or a government subscriber, and different subscribers have different requirements for service reliability. A subscriber call coupled to an AG device may be an ordinary call, or may be an emergency call such as a call for ambulance, a call for police and the like, and different calls also have different requirements for service reliability. Therefore, a key technology in the overload control for the AG device lies in that a reasonable handling mechanism and call restricting policy shall be studied to guarantee a call with high priority during an overload as far as possible.
In a first approach of the prior art, multiple overload levels are set at the AG device, and each of the levels corresponds to a certain system load. For instance, three overload levels are set, where an overload at the first level corresponds to a CPU occupancy rate of 70%, an overload at the second level corresponds to a CPU occupancy rate of 85%, and an overload at the third level corresponds to a CPU occupancy rate of 95%.
A ratio of the subscribers whose calls are restricted in the overload at the respective levels is set at the AG device. The system initiates call restriction dependent upon the ratio for call restriction when the overload of the system reaches a certain level. For instance, in the case of the overload control of three levels, 50% subscriber calls are restricted for the overload at the first level, 70% subscriber calls are restricted for the overload at the second level, and all subscriber calls are restricted for the overload at the third level. 50% or 70% subscriber calls are selected randomly and restricted during call restriction. For instance, for restricting 50% calls, five calls per ten calls are rejected, and for restricting 70% calls, seven calls per ten calls are rejected.
For the case that a subscriber coupled to the AG device acts as a calling side, when the AG detects a hook-off by the subscriber, the AG judges whether to restrict the call dependent upon a current overload level. If it is judged to restrict the call, the call is directly rejected and a busy tone or a notification tone is played to the subscriber. Otherwise, the hook-off event is reported to the MGC through the H.248/MGCP, and the call is handled in accordance with normal call flow.
For the case that a subscriber coupled to the AG device acts as a called side, when the AG receives an Add command sent by the MGC, the AG judges whether to restrict the call dependent upon a current overload level. If it is judged to restrict the call, the call is rejected in a Reply message sent to the MGC. Otherwise, the call is accepted in the Reply message sent to the MGC, and the call is handled in accordance with the normal call flow.
The first approach in the prior art may be disadvantageous in that any call is restricted uniformly dependent upon the set ratio of the call restriction during the overload of the AG device without distinguishing priorities of calls. Although this may protect the system from breakdown under the impact of a large traffic, no guarantee can be presented for a call with high priority, which results in that the call with high priority may fail even in the case of a slight overload.
In a second approach of the prior art, multiple overload levels are set at the AG device, and each of the levels corresponds to a certain system load. For instance, three overload levels are set, where an overload at the first level corresponds to a CPU occupancy rate of 70%, an overload at the second level corresponds to a CPU occupancy rate of 85%, and an overload at the third level corresponds to a CPU occupancy rate of 95%.
The call restriction policy for the overload of the AG is set at the MGC device. The call restriction policy may be call restriction as per subscriber proportion, or may be call restriction as per call priority with priority set for a subscriber or calling number.
When the AG detects its own overload, overload information of the AG is reported to the MGC in a gateway overload control packet of the H.248.11.
The AG rejects all calls during an overload at the highest level, and performs process in accordance with the normal call flow during an overload at other levels without initiation of the call restriction. Upon reception of the overload information reported by the AG, the MGC restricts calls from a calling side and a called side of the AG dependent upon the preset call restriction policy.
Though the second approach in the prior art can guarantee a call with high priority, it still has the following disadvantages.
1. The AG does not perform call restriction substantially. The call restriction primarily depends on the MGC, which poses high functional requirement as well as intercommunication requirement on the MGC.
2. The AG does not perform call restriction substantially, but reports numerous subscriber hook-off messages to the MGC, which brings a large impact upon the MGC and tends to result in the overload of the MGC.
3. Since the AG still performs process in accordance with the normal call flow during an overload (except an overload at the highest level) and there are still a large number of messages interacted between the AG and the MGC, it is not easy to alleviate the overload of the AG, thereby normal access to the service will be affected for a long period of time.